1. Field of the Invention
The present invention relates to optometers and refractometers and more particularly to a refractometer and instrument which measures spherical refractive errors using a hand-held device and ambient light. The refractometer enables a user to view a image or target a selected distance from the instrument and by the user adjusting the device to obtain a clear image enabling refractive error of the user's eye to be indicated on a linear diopter scale based on user's self-test of observing and focusing a clear image as viewed through the refractometer.
2. Description of the Prior Art
Apparatus and methods for measuring the accommodative state of an eye of an individual user is known. U.S. Pat. No. 4,997,269 discloses a Scheiner-principle pocket optometer for self-evaluation and bio-feedback accommodation training. The optometer apparatus disclosed in U.S. Pat. No. 4,997,269 includes a pinhole aperture plate having a center and a plurality of apertures in the pinhole aperture plate for viewing by the user's eye. The optometer apparatus includes a postive lens disposed near the pinhole aperture plate and has an optical axis coincident with the center of the pinhole aperture plate. The optometer apparatus further includes a scaled means inclined away from the positive lens for indicating to the subject the accommodative state of the user's eye in diopters.
U.S. Pat. No. 4,997,269 further discloses the use of a fixed positive Badal lens which is placed in a position adjacent to the pinhole aperture plate such that the optical axis of the Badal lens is coincident with the center of the pinhole aperture plate. In use, the user's eye views the optometer to observe a specific point on an inclined scaled means and to determine if a specific point on the scale is closer to or farther away from the Badal lens than a horizontal line and light which is emitted from apertures in the pinhole aperture plate such that two bundles of light with the right amount of divergence or convergence, as the case may be, cancel out the excess divergence or convergence in the eye. As a result the two bundles of light from the apertures and the light that originates from a specific point on the inclined scaled means are brought to coincidence on the retina of the user's eye. As a result, the image being viewed by the user's eye appears to be in focus and, as such, there is no stimulus for accommodation. In this apparatus, the lack of an accommodative stimulus effectively opens an accommodative control feedback loop, thus facilitating the attainment of cognitive control for the user. As a result, no mechanical movement of the optometer apparatus is required in that a user uses the optometer apparatus by relying upon the user's eye seeing an "X" image with a scale that indicates the accommodation by placement of the intersection of the "X" on the inclined scaled means.
U.S. Pat. No. 4,679,921 discloses an eye examining apparatus comprising multiple lens systems. The lens systems function to form an image of a target mark at a position near the focal point on the target mark side of a first lens unit. The focal point of the examined eye of the composite system comprising a negative lens unit, a second positive unit and a third positive lens unit is approximately coincident with the position conjugate with the pupil of the examined eye relative to the first positive lens unit. The diopter of the eye-examining apparatus is changed by moving the third positive lens unit and the change of the axis and degree of astigmatisn are achieved by rotating the astigmatism system.
U.S. Pat. No. 843,503 discloses an optometer having a relatively moveable eye and object lenses and a non-linear bent or crooked scale to provide diopter readings of the subject's eye.
U.S. Pat. No. 735,460 discloses a refractometer having an adjustable lens and a rotary scale for indicating the convex and concave required.
U.S. Pat. No. 712,719 discloses a device having a removeable objective lens for measuring hypermetropia, myopia and astigmatism on a linear scale when the properly selected lens is placed into the device as an objective lens which is then moved relative to the eye. The results are shown by a pointer over the linear scale.
U.S. Pat. No. 698,833 discloses an optometer having a moveable target for testing range of accommodation in diopters on a linear scale.
U.S. Pat. No. 667,973 discloses an optometer having either a monocular or binocular optometer having lenses which are concurrently both longitudinally moveable along the optical axis and which are rotatable around the optical axis for providing an unmagnified image and a linear scale.
U.S. Pat. No. 683,973 discloses an optometer having a moveable negative objective lens, compound lens and a linear scale.
U.S. Pat. No. 268,316 discloses an optometer having a moveable objective disk and a rotary diopter scale.
In addition, in the field of ophthalmogy it is known to utilize sophisticated optometers apparatus for collecting refractive data for a patient to measure the spherical refractive error of a patient's eye. Typical of such apparatus is a Topcon Autorefractor (Model RM-A2000). Also, standard subjective refractions are accomplished with trial frames and lens.
Historically, optical system for measuring refractive error of the human have indicated a wide variety of refractometer using complex spherical and cylindrical lens. These include optical devices with variable cylindrical power such as the "Stokes Lens"; tilting spherical lens and complex, two cylindrical lens.
The use of a Badal lens in an optometer device is also known in the art. The term "Badal Lens" is used to describe an optometer having a single lens providing constant angular magnification, constant image brightness and a linear scale throughout its diopter range. The "Bandel Lens" and other known optometer devices are described in an article entitled "AN OPTICAL SYSTEM WITH CONTINUOUSLY VARIABLE SPHERECYLINDRAL POWER" by David L. Guyton and Hewitt D. Crane which appeared at pages 26 through 30 in SPIE, Vol. 141, ADAPTIVE OPTICAL COMPONENTS, 1978 (the "GUYTON et al Reference"). The Guyton et al reference contains a detailed description of the optometer principle which uses a single, converging lens which is fixed in position a distance equal to its focal light from the spectacle plane. Light arrives at this spectacle plane with a vergence of different amounts (plus, zero or minus) depending upon the axial position of the target T on the far side of the lens. A linear scale in diopters may be positioned on the far side of the convergence lens indicating the power of spectacle lens which this arrangement simulate in the spectacle plane.